The present invention relates to a method and an apparatus for controlling the amount of a scanning electron beam of an image pickup tube having a reduced image lag and a diode type electron gun.
In a photoconductive image pickup tube, an electric charge pattern corresponding to a scene illuminance is generated on a photoconductive layer, and then the surface of the photoconductive layer is scanned by an electron beam emitted from an electron gun, so that the electric pattern is subjected to a sequential discharge and that a charge current corresponding to the discharge is externally obtained as a signal. For such a photoconductive image pickup tube, it has ben desired to reduce the image lag and to expand the range of light quantity.
The image lag is produced because the electric charge generated for a scene cannot be completely discharged by a beam scanning operation and the remaining charges are also read in the next and subsequent scanning operations. Particularly in an image pickup tube using a blocking type photoconductive layer, the image lag is mainly caused due to a capacitive signal lag associated with a time constant determined by the product of the static capacitance of the photoconductive layer and the beam resistance related to the scanning electron beam.
To lower the beam resistance, it is indispensable to narrow the velocity distribution of electrons forming the electron beam. The electrons emitted from a cathode have a velocity distribution according to the Maxwellian distribution. It has been known that the current density of the beam is increased in a process for generating a narrow beam and that the velocity distribution becomes broader due to the energy relaxation phenomenon due to coulomb force interaction between the electrons. This is called Boersh effect and the expansion rate of velocity distribution is known to be substantially in proportion to J(Z).sup.1/3, where the current density of the electron beam on its axis is J(Z). Consequently, to provide an image pickup tube with a low image lag, the increase of the beam current density must be minimized to the possible extent. To this end, there has been proposed a diode type electron gun in which a first grid electrode opposing a cathode is set to a positive voltage with respect to the cathode, which emits electrons in a direction parallel to the axis of the image pickup tube, thereby preventing formation of a crossover having a high current density (for example, Japanese patent application Laid-Open Nos. 50-39869 and 54-129871 and U.S. Pat. Nos. 3,894,261 and 4,388,556).
FIG. 12 is a schematic diagram illustrating an example of a diode type electron gun (This electron gun is proposed in the pending U.S. patent application Ser. No. 755,014.) satisfying the gist described above which includes a cathode 1 and a first grid electrode 2 having an aperture 2a and being supplied with a voltage E.sub.1 positive with respect to the cathode 1. Reference numeral 3 denotes a second grid electrode having an aperture 33 smaller than the aperture 2a of the first grid electrode 2 and being supplied with a positive voltage E.sub.2 higher than the voltage E.sub.1. Reference numeral 4 indicates a generated electron beam. When the voltage E.sub.1 is variable, the electron beam 4 can be changed from a laminar flow electron beam illustrated with dashed lines into a crossover beam depicted with solid lines. FIG. 13 is a graph illustrating the relationships between the first grid voltage E.sub.1 and the beam current generated by the electron beam received by a photoelectric conversion layer (photoconductive target). Point A is the ordinary operating point where the laminar flow electron beam is formed and the image lag can be reduced, whereas the crossover is developed at point B. As described above, the image pickup tube having the electron gun of FIG. 12 enables to obtain a large beam current because of the variable first grid voltage E.sub.1, thereby achieving the Automatic Beam Optimizer (ABO) operation in which the amount of an electron beam is controlled corresponding to the scene illuminance.
The U.S. Pat. No. 4,540,916 proposed a diode type electron gun in which an intermediate electrode is provided between the first and second grid electrodes so as to be especially suitable for the ABO operation.
The ABO operation has been described in the U.S. Pat. Nos. 3,975,657; 3,999,011; and 4,151,552.
FIG. 14 is a simplified circuit diagram illustrating the conventional ABO circuit comprising an image pickup tube 10 having a diode type electron gun as shown in FIG. 12. According to the conventional method, a signal current obtained from a photoconductive target 11 through an electron beam scanning is converted into a voltage by use of a resistor 12. The voltage is then amplified by an amplifier 13 and then the resultant voltage is added to a reference bias voltage 15 by an adder 14. The output voltage of adder 14 is in turn applied to a beam current control electrode (first grid electrode) 2. The following relationships hold in this circuit. EQU V.sub.S =-I.sub.S .multidot.R.sub.S ( 1) EQU V.sub.0 =B.multidot.V.sub.S ( 2) EQU E.sub.G1 =V.sub.0 +V.sub.01 ( 3)
where, V.sub.S is an input signal voltage of the amplifier 13, I.sub.S is the signal current, R.sub.S is a resistance value of the resistor 12, V.sub.0 is an output signal voltage of the amplifier 13, B is the amplification factor of the amplifier 13, E.sub.G1 is the beam current control electrode voltage (first grid electrode voltage), and V.sub.01 is the reference bias voltage.
Assuming the ratio between the beam current control electrode voltage and the beam current to be gm.sub.1, the following relationships are satisfied. EQU I.sub.B =-gm.sub.1 .multidot.E.sub.G1 ( 4) EQU I.sub.01 =-gm.sub.1 .multidot.V.sub.01 ( 5)
where, I.sub.B is the scanning beam current and I.sub.01 is the bias value of the scanning beam current.
From the five expressions described above, I.sub.B can be expressed as follows: EQU I.sub.B =A.multidot.I.sub.S +I.sub.01 ( 6)
where, A=gm.sub.1 .multidot.R.sub.S .multidot.B. Expression (6) indicates the control characteristic of the scanning beam current of the circuit shown in FIG. 14. The ideal ABO operation is possible when A=1 is satisfied. The circuit, however, has three problems.
The first problem is that if a capacitive coupling as denoted by reference numeral 17 exists between the first grid electrode 2 and the photoconductive target 11 of the image pickup tube 10, a positive feedback circuit is formed as shown in FIG. 15, which leads to an oscillation. This oscillation occurs because the (negative) polarity of the signal voltage applied to the first grid electrode 2 is the same as that of the signal charge generated at the photoconductive target 11 and hence the signal mixed by the capacitive coupling is added at the photoconductive target 11 with the same polarity.
The second problem is that when the beam control circuit of FIG. 14 is set to a state in which the beam is insufficient during the operation, an oscillation may take place depending on the circuit condition. In this state, the signal current is independent of the information of light and increases in proportion to the beam current, and hence the circuit of FIG. 14 becomes identical to a positive feedback circuit. Assuming the proportional constant between the signal current and the electron beam current to be F, then EQU I.sub.S =F.multidot.I.sub.B ( 7)
results. From the expressions (6) and (7), the following relationship is extracted. ##EQU1##
The oscillation condition of the positive feedback circuit is determined by the denominator of the expression (8), and the oscillation does not occur for AF&lt;1. Although the value of F varies depending on the characteristics and state of the photoconductive layer (photoconductive target) 11, this value is almost one; and to prevent the oscillation, it must be considered to satisfy the condition of A&lt;1 or to avoid the insufficient beam state.
The condition of A&lt;1 can be realized by lowering the amplification factor B of the amplifier 13; however, since the value of gm changes with the voltage applied to the first grid electrode 2, the value of B must be determined for the maximum value of gm. As a result, there appears a disadvantage that the amplification factor becomes insufficient and the control range is reduced at the operating point where the value of gm is minimum.
The third problem is that when the amount of an electron beam is increased, the beam resistance soars, and hence the image lag is increased. In the diode type electron gun of FIG. 12, if the first grid voltage E.sub.1 is changed to increase the amount of the electron beam, then the electron beam changes from the laminar flow of electron beam illustrated with dashed lines to the crossover beam depicted with solid lines. The crossover beam has a high current density and consequently develops a high beam resistance, which leads to a disadvantageous result that the image lag is increased.